I do not understand why there is a Book of Life.
The Book of Life is a record written by God before the creation of the world, listing who will live forever in the kingdom of heaven. God knew before the world was created who would be saved and who would suffer eternal damnation.
It seems He knew who would accept Jesus and who would not.
What about "free will"? Has our decision to accept Christ been pre-ordained or pre-destined? Did God know what we would choose before we made a choice? But that is another rabbit hole for another thread.
More interesting is why God would need a book. He knows everything. Why does he need to look this up in a book when Judgement Day happens?